Microbiologic clearance in macrolide-resistant Bordetella pertussis: An infant treated with sulfamethoxazole-trimethoprim - A case timeline.

Amid a large-scale resurgence of pertussis in Japan in 2025, concern is growing about the spread of macrolide-resistant Bordetella pertussis (MRBP)-prevalent in China-posing a clinical challenge, particularly in infants. We describe a term 6-week-old unvaccinated infant with life-threatening MRBP. Diagnosis was confirmed by culture and multiplex PCR. The isolate harbored the 23S rRNA A2047G mutation. Daily microbiology (Gram stain, culture, quantitative PCR) was performed on nasopharyngeal swabs and lower-airway aspirates. The infant received empiric azithromycin and a 14-day course of sulfamethoxazole-trimethoprim (SMX-TMP) and recovered without sequelae. Nasopharyngeal cultures became negative by Day 2 of SMX-TMP therapy, and lower-airway aspirate cultures by Day 3. Nasopharyngeal quantitative PCR was below the detection limit by Day 7. Lower-airway quantitative PCR declined through Day 7, and sampling ceased after extubation. These observations suggest that bacteriologic clearance with SMX-TMP may occur on a time frame similar to that used to discontinue precautions for macrolide-susceptible disease, commonly five days after starting effective therapy. During the current resurgence in Japan, transmission appears concentrated among school-aged children, whereas the national vaccination program targets only those under two years of age. Introducing booster doses at 5-6 and 11-12 years may help reduce school-based transmission. In addition, maternal booster vaccination during pregnancy could protect young infants through the transplacental transfer of maternal pertussis antibodies.